Auckland Mayor Len Brown is offering $250 million of the city's money to kick-start the $2.86 billion underground rail project before the Government starts contributing.

He has told Prime Minister John Key his council will pay for an "early works" programme from next year to get the project out of its starting blocks at Britomart and under much of Albert St.

The council, through Auckland Transport, has already spent more than $100 million on property purchases and other route preparation work, and has included $193 million for the "transformational" project in its draft budget for 2014-15.

But North Shore councillor George Wood is aghast that Mr Brown should be making such a sweeping offer to the Government without having reached an agreed formula for paying Auckland's half share of the rail project.

"If you're going to bulldoze it, and say we're going to do it, I don't think it's good budgeting and it's not prudent stewardship of the funds of Auckland Council," Mr Wood told the Herald yesterday.

Mr Brown told the Prime Minister in a letter - obtained by the Herald - that his offer was based on a Government commitment to pay for half of the project from 2020, or earlier if Auckland can meet tough rail patronage and employment growth targets set last year by Mr Key.

"Effectively, council would be underwriting the Government's contribution until 2020 (or an agreed earlier date)," Mr Brown wrote, in preparation for a meeting he held with Mr Key last week.

After announcing Government support for the project last year, Mr Key said: "We are bluntly committing to paying for a decent chunk of it - 50-50."

But Mr Brown said last night the council was waiting for Government confirmation of a 50 per cent contribution.

In his letter to Mr Key, he listed a string of private sector projects likely to be affected by construction of the 3.5km rail link from Britomart to Mt Eden.

They included a $300 million-plus redevelopment of the Downtown shopping centre above the route, and the convention centre which Sky City intends building for the Government in return for being allowed to install extra gambling machines.

Mr Brown said an early start to the rail project would minimise disruption and provide "a more effective and investment-friendly approach to the overall development of Auckland's CBD".

Precinct Properties wants to start rebuilding the Downtown centre next year into a possible 41-storey tower. It will co-ordinate foundation work with excavations for a "cut and cover" section of two rail tunnels between Britomart and a new underground station near Aotea Square.

Mr Brown said Aucklanders had shown overwhelming support for the rail project, and the private sector was making investment plans around it.

"We are saying to the Government, there is a big head of wind coming in behind this project from the private sector, and do we want to hold them up while we continue to dally, or do we want to move."

A spokeswoman for Mr Key said he had indicated to Mr Brown he would ask officials to look at the Mayor's proposal, but the Government's position was unchanged.